The present invention relates to gas turbine engines for aircraft and, more particularly, to gas turbine engines each coupled to a corresponding auxiliary engine.
Gas turbine engines as continuous combustion, open Brayton cycle internal combustion engines have come to dominate as the power plants for larger, faster aircraft to essentially the exclusion of reciprocating engines, or internal, intermittent combustion engines, earlier used as power plants for these kinds of aircraft. This is largely because of the greater power-to-weight ratio of gas turbine engines versus internal combustion engines, especially in large horsepower engines, or, more appropriately, large thrust engines in which those large thrusts are provided with a relatively small, and so smaller drag, frontal area engine structures relative to reciprocating engines. Gas turbine engines generate such large thrusts for propulsion, or horsepower for engines with an output shaft, by combining large volumes of air with large amounts of fuel, and thereby form a jet of large velocity leading to the capability to provide desired speedy flights.
In addition to providing thrust, such gas turbine engines have operated integrated drive generators to generate electricity for the aircraft and for the engine electronic controls. The amount of electricity needed for these purposes in the past has tended to be relatively modest typically in the range of a few hundred kilowatts of electrical power but, with recently arriving new aircraft, exceeding a megawatt of power. However, there are some aircraft, usually for military uses, that have come to have needs for much larger amounts of electrical power either on a relative basis, the electrical power needed relative to the capability of the gas turbine engine available, or on an absolute basis with power needs significantly exceeding a megawatt. Furthermore, such demands for electrical power in military aircraft often occur at relatively high altitudes and often occur unevenly over relatively long time durations of use, that is, large peaks repeatedly occur in electrical power demand in the course of those long use durations.
Corresponding attempts to obtain the added power from the typical aircraft propulsive system, the gas turbine engine, that are needed to operate the concomitant much larger capacity electrical generators, either on a relative or absolute basis, will subtract significantly from the thrust output of the available turbine or turbines. Making up that thrust loss in these circumstances by operating such available turbine engines so as to increase the thrust output thereof causes the already relatively low fuel use efficiency during flight to decrease significantly, which can severely limit the length of otherwise long duration uses, and also brings those engines closer to becoming operationally unstable.
One alternative to using the gas turbine engine as the sole source of power to operate an electrical power generator is to add in the aircraft a further intermittent combustion internal combustion engine, such as gasoline engines operating on the any of the Diesel, Miller, Otto or Wankel cycles. Such engines can operate with a fuel efficiency on the order of seventy percent (70%) better than that of a continuous combustion (Brayton cycle) internal combustion gas turbine engine. At high altitudes, internal combustion engines of all kinds face the possibility of limited power output because of the relatively small air pressures there limiting the chemical reactions of oxygen with hydrogen and oxygen with carbon in the burning of the engine fuel in the engine combustion chamber or chambers. This can be solved for gas turbine engines by providing therein very large air flows through use, typically, of axial flow compressors usually in two stages with both a low compression compressor followed along the fluid flow path through the engine by a high compression compressor. This arrangement provides at least enough compressed air to the subsequent combustor to sustain the desired combustion process therein and a mass of airflow sufficient to combine with enough fuel to provide the energy needed to overcome the aircraft drag at the speed and altitude intended for operation.
However, such compressors can provide considerably more compressed air than the minimum needed for this purpose thereby allowing some of this compressed air to be delivered through an air transport duct to the air intake of an intermittent combustion internal combustion engine so that, in effect, the compressors of the gas turbine engine serve as a very capable supercharger for that intermittent combustion engine. Thus, this intermittent combustion engine can be operated at the same relatively high altitudes at which the gas turbine engine propelling the aircraft operates while this turbine engine is also supplying compressed air to that intermittent combustion engine. There, depending on the values selected for the peak air intake pressure and engine compression ratio, the intermittent combustion engine can be used as a power source for an electrical power generator that can generate much greater amounts of electrical power than can one powered by a gas turbine engine.
In such aircraft so equipped with a gas turbine engine used as a supercharger for an accompanying intermittent combustion engine while also propelling the aircraft, the amount of electrical power needed at any time during flights thereof substantially determines the amount of torque needed to be supplied by the intermittent combustion engine to the electrical power generator. The amount of torque generated is determined by the amount of fuel supplied to the combustion chambers of the intermittent combustion engine, and there is a corresponding amount of air that must also be supplied to those chambers to support the desired combustion therein. At some altitudes and certain other flight conditions and for some amounts of electrical power concurrently demanded in the aircraft, the amount of compressed air supplied by the gas turbine engine through an air transport duct to the air intake of the intermittent combustion engine is more or less optimal. In other circumstances, difficulties arise because of aircraft operations being required at different altitudes with the attendant different air pressures, because of different gross weights due to different flight missions and the consumption of fuel during flights with the attendant differing operating powers that must be provided by the gas turbine engine, and the like.
These differing circumstances change the pressure of the compressed air being delivered by the gas turbine engine air compressor to the intermittent combustion engine, and so could exceed its pressurized air containment capability or change its torque generating capability and, correspondingly, the amount of electrical power generated. In these other circumstances, the intermittent combustion engine might be overpressured at its air intake because the compression ratio of the engine could increase the pressure of the already compressed air supplied thereto to the point of some containment component bursting. Alternatively, the intermittent combustion engine might be insufficiently pressured there to result in incomplete combustion in the combustion chambers thereof and so a reduced output torque. Thus, there is a desire to better match the air pressure at the air intake of the intermittent combustion engine to varying flight and electrical power demand conditions.